The present invention relates to a cutting insert for chip forming machining tools, in particular milling cutters, such as face millers. The cutting inserts are preferably produced by form pressing and sintering of an insert forming powder. The cutting insert comprises an upper chip surface, a lower, planar bottom surface which is adapted to be located in abutment with a cooperating bottom support surface of the machining tool, and at least three side surfaces extending between the upper and bottom surfaces. At least one of the side surfaces adjoins the upper chip surface along an edge which constitutes a cutting edge.
Such inserts are more and more produced by a direct pressing method during which a cemented carbide powder is first shaped into a desired form by a pressing tool adapted for the purpose and subsequently is given final strength by sintering in an oven at temperatures above 1000.degree. C. The pressing operation has been sophisticated over the years and is today so well defined that the process provides good possibilities of shaping the cutting edge, adjacent chip forming surfaces, possible reinforcing chamfers and clearance surfaces with great exactitude. Moreover, even the subsequent shrinkage that occurs during sintering can be included into the computation for the pressing tool dimensioning.
Today's cutting geometries tend towards sharper and sharper rake angles, i.e., more and more positive rake angles. As is well known, this brings about several advantages, such as a small cutting force and thereby a low energy consumption, a well-defined cutting edge for high dimension precision, as well as greater liberty when selecting the clearance angle, with maintained positive cutting geometry. The drawback is naturally that the cutting edge angle becomes sharper and thereby weaker, which can cause chipping in the cutting edge, particularly at higher work loads such as at higher feeds and/or higher cutting depths. The most exposed part of a usual indexable insert is the cutting edge end, i.e., the comer of the insert that is in engagement with the work piece. Therefore, the end or the immediate proximity thereof is most susceptible to wearings such as chipping and breakage. Consequently, it would be desirable to reinforce the cutting edge comer so that it can withstand high work loads, while simultaneously retaining a cutting geometry as positive as possible, the latter for the purpose of attaining a cutting force as small as possible. Particularly at larger cutting depths, where a larger part of the cutting edge is in engagement with the work piece, a chip angle as positive as possible is desirable, since the cutting force increases with increased cutting depth. At small cutting depths, the effect of the ordinary milling machines is normally sufficient.
The present invention has in a surprising and unexpected manner succeeded in satisfying both of the above indicated criteria although, seemingly incompatible, of a strong cutting comer portion and a most positive as possible chip angle. This result has been achieved by designing the chip surface with a helically twisted form in such a manner that the rake angle increases with increasing cutting depth.
A cutting insert for chip forming machining tools according to the present invention is formed with a chip surface whose chip or rake angle increases with increasing cutting depth. In this manner the cutting edge becomes stronger closer to the operative cutting insert comer, where the work load is the largest. Simultaneously, the rake angle increases and the cutting forces diminish towards increasing cutting depth, where the specific work load is smaller.
Indexable cutting inserts with twisted chip surfaces along the cutting edges are already known per se, see for example EP-A-239 045 and EP-A-392 730. However, both of these publications disclose inserts whose chip surfaces are twisted in a directly contrary manner relative to the present invention, i.e., the rake angles nearest the cutting insert comer in these publications, have a maximally positive chip angle, while at the "cutting depth maximum," these publications disclose the least positive rake angle.
Consequently, these known inserts are impaired by two drawbacks which the present invention has managed to eliminate. On the one hand, the known inserts suffer a weakened cutting comer portion and on the other hand the specific cutting force increases with increasing cutting depth. Further, the two EP publications both disclose the feature that the clearance surface shall be helically formed, for the purpose of holding the cutting edge angle constant. Even if this constant angle in itself entails the advantage that the clearance angle can be kept substantially constant in relation to the work piece at positive axial inclinations of the insert, this prior art design nevertheless also results in the disadvantage of a further weakening of the cutting edge in comparison with an indexable insert whose side faces are perpendicular or slightly inclined with a constant obtuse angle in relation to the planar bottom surface.
The inserts according to the present invention have turned out to be surprisingly capable when machining hard materials, as well as workpieces with many cavities, for example, aluminum-based profiles in the aeronautic industry. The inserts have even advantageously been used on a steel with a low carbon content without any tendency of burring. Thanks to the advantageous positive cutting geometry, the inserts according to the present invention have also been successfully used in connection with weaker milling machines.